Many of today's lighting needs are fulfilled using light-emitting diodes (LEDs). Indeed, LEDs have become ubiquitous throughout a wide range of applications. Commonly, blue-emitting LEDs are used and are coated with yellow or orange or red phosphors that serve to down convert the higher energy (e.g., near blue) photons to lower energy (e.g., near red) photons. In some applications, the yellow and/or orange and/or red phosphors are visible to the naked eye, which can detract from the cosmetic appearances. In certain applications (e.g., flash units in cameras, cell phone flash units, automotive headlamps, etc.) the phosphors are purposely covered with another layer that diffuses incident light sufficiently so that the undesired colors of the phosphors are not visible. In some cases, even when the additional diffusing layer completely covers the phosphors, the yellow/orange/red color “peeks” through. One approach to reduce peek-through is to make the diffusing layer thicker or denser. At some point the thickness or density of the diffusing layer produces enough scattering of incident light that there is no visible peek through of the yellow or orange phosphors.
Unfortunately, making the diffusing layer thicker and/or denser also reduces the operating efficiency of the LED device. More specifically, legacy uses of thicker and/or denser diffusing structures (e.g., scattering layer thicknesses, scattering particle density, etc.) that diffuse enough to reduce or eliminate peek-through also has the undesired effect of diffusing, scattering or absorbing light from the LED when in operation, thus reducing the efficiency of light production.
Improvements are needed.